Joining between metallic or thermoplastic and composite components is currently approached in a number of ways, each with its own limitations.
The use of fasteners is commonplace but tends to result in de-lamination around fastener holes. Fastener holes are often difficult to drill in composites and significant reinforcement around fastener holes may be required, leading to increased weight. Fastened joints tend to be particularly weak in the pull-through direction (that is, the direction of axial load through the fastener). As such, fastened joints are not well suited to many aerospace applications.
Adhesive bonds are an increasingly common means of joining metallic components to composite laminates, however these perform poorly in peel, tension and cleavage, and tend to fail with little or no warning. Their weakness in peel and in tension makes bonded joints similarly limited in their application within conventional aerospace structures. Any mitigation for the poor performance in peel or tension tends to result in large bond surface areas, with the associated weight penalties that go with this.
WO 2004/028731 A1 describes a method by which surface features are generated by using a ‘power-beam’ such as an electron beam, in order to ‘flick-up’ surface material on a metallic component to sculpt protruding features that are intended to increase bond surface area and improve bond strength when incorporated into the matrix of a co-cured laminate. The displacement of surface material to create the protruding features is likely to generate crack initiators that will adversely affect the fatigue life of the component. Also, it is difficult to optimise the profile and shape of the surface features.
WO 2008/110835 A1 describes a method by which surface features are “grown” on a bond surface of a component in a series of layers by an additive fabrication process. The profile and shape of the surface features can be controlled so as to tailor the performance of the joint, particularly in tension and peel. Each surface feature may have a pointed tip such that the surface features may easily be embedded into a series of laminate plies draped successively over the bond surface. Each surface feature may also include a part having an overhanging edge such that the component may be used in a joint that requires enhanced pull-off (tensile) strength.